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    <script>
        /* &&与  ||或   !非 
        && 两边都为true 结果才为true
        || 只要有一边为true结果就为true
        ! 取反
        */
        var a=20;
        var b=30;
        var c=40;
        var d=50;
        console.log(a<b && c<d)
        console.log(a<b && c>d)
    </script>
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